自我加入与其他表上的内部联接

Question

我有一个数据库与3表记录，类别，关系。

records (id, lat, lng) 
categories (c_id, c_value) 
relational (r_id, c_id) 

records 
id | lat  | lng 
---------------------- 
1 23.57258 -35.28412 
2 23.54855 -35.18881 
3 23.74128 -35.17469 

categories 
c_id | c_value 
--------------- 
100 groceries 
101 bags 
102 drinks 

relational 
id | c_id 
------------ 
1  100 
1  102 
2  101 
3  100 

的relational.r_id = records.id和relational.c_id = categories.c_id 我想从不同c_value记录采取对，所以我想做个自我记录中加入并在内部联接类别和关系。 我做了这个没有自我的记录

SELECT id, lat, lng, c_value 
FROM records 
JOIN relational 
    ON records.id = relational.id 
JOIN categories 
    ON relational.c_id = categories.c_id 
WHERE c_value = "V1" 

加入我想是这样的，但没有奏效。我对R1.c_value有问题。

SELECT R1._id, R1.lat, R1.lng, R1.c_value, R2._id, R2.lat, R2.lng, 
     R2.c_value 
FROM records R1, records R2 
JOIN relational 
    ON records.id = relational.id 
JOIN categories 
    ON relational.c_id = categories.c_id 
WHERE R1.c_value = "groceries" AND R2.c_value = "bags" 

你知道我该如何结合这3个连接以从记录中取出2行与其他表的标准？ 我希望有一个这样的输出： 对于“杂货”和“袋”为c.value

1 | 23.57258 | -35.28412 | groceries | 2 | 23.54855 | -35.18881 | bags  

Answer 1

我终于找到了我想要的东西。我的查询应该是这样的。无论如何，感谢您的帮助和想法。

SELECT r1.id, r1.lat, r1.lng, c1.c_value, r2.id, r2.lat, r2.lng, c2.c_value 
FROM records r1 JOIN relational rl1 ON rl1.id = r1.id 
JOIN categories c1 ON rl1.c_id = c1.c_id 
JOIN records r2 JOIN relational rl2 ON rl2.id = r2.id 
JOIN categories c2 ON rl2.c_id = c2.c_id 
WHERE c1.c_value = "groceries" AND c2.c_value = "bags"  

Answer 2

您可以自行加入records表categories和relational这样一起：

SELECT r1.id, r1.lat, r1.lng, r2.id, r2.lat, r2.lng, c.c_value 
FROM records r1 JOIN relational rl ON r1.id = rl.r_id 
JOIN records r2 ON r2.id = rl.r_id 
JOIN categories c ON rl.c_id = c.c_id 
WHERE r1.id <> r2.id;